Sintered Neodymium-Iron-Boron (Nd—Fe—B) magnets have the highest energy product among current permanent magnets. However, sintered Nd—Fe—B magnets have a relatively low Curie temperature of about 312° C., which may prevent them from being used in some high temperature applications, such as electric vehicles and wind turbines. Several approaches have been taken to improve the thermal stability of sintered Nd—Fe—B magnets. Alloying is one approach that has been investigated. Cobalt substitution for iron may increase the Curie temperature; however, it may also decrease the anisotropy field and therefore the coercivity of the magnets. Another approach that has been tried is the substitution of Dysprosium (Dy) or Terbium (Tb) for Nd. Addition of these heavy rare earth elements can significantly increase the anisotropy field of the hard magnetic R2Fe14B (R=rare earth) phase. Although the coercivity of sintered Nd—Fe—B magnets can be effectively increased by such substitution, the antiparallel coupling between these heavy rare earths and the Fe spin moments in Dy—Fe and Tb—Fe leads to a significant decrease in saturation magnetization. In addition, Dy and Tb are much more expensive and much less abundant than Nd.
In addition to alloying, another approach to increasing the thermal stability of Nd—Fe—B magnets is the forming of a hybrid magnet, which is a mixture of different permanent magnets with magnetic properties compensating for each other. For example, one magnet with high magnetization and another with high thermal stability. Due to the dipolar interaction, the thermal resistance of the high magnetization material can be improved by the high thermal stability material. In previous research, Samarium-Cobalt (Sm—Co) alloys have been used as high thermal stability materials, in particular SmCo5 and Sm2Co17, for their much higher Curie temperature compared with Nd2Fe14B.